The invention relates to a device for cleaning and cooling a liquor gun, said gun comprising at least a liquor tube for feeding liquor, such as black liquor, into a furnace of a chemical recovery boiler.
In chemical cellulose pulping roughly 35-80% of raw wood material is processed to chemical pulp, so-called “yield”. In chemical pulping the yield is typically 35-55%. The rest of the wood exits the chemical pulp production unit in form of waste liquor that contains, in addition to wood-originating substances, also chemicals used in chemical pulping, which are regenerated for re-use in a so-called recovery unit of the chemical pulp mill. The most common pulp production process is the so-called kraft-process, which generates waste liquid called black liquor. The recovery unit mainly comprises the increase of dry solids in the waste liquor to 65-85%, and even higher, combustion of thus obtained strong waste liquor in the chemical recovery boiler, recovery of chemicals released and regenerated in connection with the combustion and processing of said chemicals for further chemical pulp production. In older production plants the dry-solids content of waste liquor may have remained below 65%, which complicates efficient combustion of waste liquor and chemical recovery. The sum of the mass fractions of the dry-solids content and the water in the waste liquor entering the combustion is very near to 100%.
In waste liquor combustion, the waste liquor is injected in form of droplets into a furnace, where the liquor droplets dry, pyrolyze, mix with oxygen carriers and burn. Oxygen carriers include oxygen, water vapor and carbon dioxide. Part of the droplets falls onto the bottom of the furnace, to a so-called char bed and burns there. The main part of in-organic matter exits the furnace via so-called smelt spouts located at the lower part thereof.
Feeding of waste liquor into the furnace and the injection is carried out by means of liquor guns. A liquor gun is in principle a metal tube connected to a piping, wherethrough strong black liquor is supplied from the preheaters to the boiler. The end of the liquor gun in the furnace is provided with a nozzle through which the black liquor is sprayed into the furnace and the design thereof varies depending on the size of the boiler and the manufacturer. The primary duty in the spraying is that it is carried out as symmetrically as possible and that the obtained droplet size is correct.
Liquor guns are typically pressure-dispersing; in some case steam or pressurized air-dispersing. A pressure-dispersing liquor gun typically has a tubular portion, inside which the waste liquor is taken from a liquor distribution system into the furnace, and the end of the tubular portion is provided with a nozzle. The nozzle can be just a nozzle, a nozzle provided with a deflector plate (a so-called spoon nozzle) or a centrifugal type of nozzle. The construction of the nozzle itself can be very simple: for instance, a “hole” in an end plate of said tube or said tube throttled by the deflector plate, which hole or throttling transforms the static pressure of liquor into dynamic pressure. FIG. 1 illustrates a principal view of a nozzle provided with a deflector plate, wherein the deflector plate i.e. spoon cuts a part of the nozzle opening, i.e. throttles the liquor flow. The waste liquor 1 enters the combustion in tube 2 from outside the furnace, from direction 5. The deflector plate 3 forms a throttling point 4 in the tube. The liquor exits the deflector plate 3 in form of a thin plate-like structure 6, which is finally scattered into smaller structures called droplets. Fouling material 11 falls and accumulates on the liquor gun
In modern boilers, the liquor is fed into the furnace horizontally or directed slightly downwards. In a liquor gun based on a centrifugal type of nozzle at least part of the liquor jet is directed sloping more downwards than in other types of waste liquor guns.
In the furnace of a recovery boiler the inorganic matter is in melted form. This causes strong corrosion in un-cooled steel surfaces, as well as deposition and fouling on the furnace walls and also e.g. in the liquor guns. Corrosion is formed also due to accumulation and ignition of unburned waste liquor on the surface of the liquor gun, which causes high temperature and corrosive conditions. Both fouling of the liquor guns and corrosion deteriorate the liquor spraying process, which has an adverse impact in view of the operation of the furnace. The effect of fouling is prevented by means of regular cleaning of the liquor guns, so-called “scrubbing” that can be done by a person, or the cleaning can take place by means of an appropriate mechanical device. The mechanical device is typically a means moving reciprocally at regular intervals, which device has a shaft and a scraper head following the tubular portion and the nozzle part of the gun in a spring-like manner. One solution of this kind is described in U.S. Pat. No. 6,478,235.
Typically, one of the major disadvantages of liquor guns is their low resistance. Due to corrosion, the life-time and thus replacement interval is from a few days to a number of weeks. The low resistance of liquor guns is due to extremely corrosive conditions and high temperature in the furnace and its surroundings, which conditions stress the wall of the gun's metal pipe. The operation of a recovery boiler has been described e.g. in publication Vakkilainen, Esa, K. Kraft Recovery Boilers—Principles and practice. Finnish Recovery Boiler Committee r.y. 2005: The spraying and combustion of liquor is described starting from page 9-1.
Attempts have been made to prevent the above described problems relating to fouling and corrosion by means of various solutions. FI-patent application 20012500 describes a cleaning device for cleaning the liquor guns of a recovery boiler. Around the shaft of the liquor gun, one or more steam pipes are arranged, steam flowing from an end of the pipe, which is arranged closer to the liquor nozzle, around the shaft of the liquor nozzle located inside the furnace. In JP-patent application 1229890, moist steam is sprayed from outside the liquor gun to the end of the gun (the nozzle part). In JP-publication 2000256979 the zone of spraying of moist steam has been widened to relate to an opening in the furnace wall surrounding the nozzle, but still the question is about using moist steam from outside the liquor gun. A known method practiced e.g., in Japan is to lead water from the outside to the shaft of the liquor gun and thus obtain a cooling and cleaning effect.
The above described solutions have used either water or steam for cooling and cleaning the liquor gun. An essential drawback of these devices is limited cleaning and/or cooling effect obtained thereby. The effect does not extend efficiently to the whole liquor gun, especially not to its outer end on the side of the furnace. E.g. when using a deflector plate, known liquor guns are devoid of efficient cleaning and cooling of the deflector plate, which are essential factors for that kind of gun, when the aim is good controlling of the combustion in the furnace.
The drawbacks described above have been partly compensated for by increasing the amount of water or steam to the gun, but a more significant improvement is needed.